Abstract: The phaseshifts induced by thermal effect and pressure difference between outersurface and innersurface of plane laser window are analysed. The phaseshifts caused by stressbirefringence are discussed in detail. Distributions of temperature increment, stress, phaseshifts, Strehlratio and depolarization are calculated．It is indicated that window deformation and refractive index changing with temperature are primary factors of phaseshifts and result from nonuniformity of temperature increment, making beam quality declining. Stressbirefringence is related with Miller indices of laser window surface, which results in depolarization and also affect beam quality.